Filter medium for an aquarium



FIG. 1 is a perspective view of a filter medium for an aquarium showing my new design;

FIG. 2 is a front elevational view thereof, the rear view being identical;

FIG. 3 is a right-side elevational view thereof, the left side being identical;

FIG. 4 is a top plan view thereof, the bottom view being identical;

FIG. 5 is a cross-sectional view thereof, taken on line 5--5 of FIG. 4; and,

FIG. 6 is a perspective view thereof, on a reduced scale, the broken line drawing of an aquarium is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a filter medium for an aquarium, as shown and described. 